Photography, in general, involves the production of images through the action of radiant energy. A common photographic method makes use of silver halide crystals which are dispersed in a protective medium on a film backing, and which crystals are sensitive to radiant energy. With exposure to a radiant energy image, the exposed crystals become more sensitive to reduction. The exposed film is developed by reduction of the exposed silver halide grains, but not the unexposed grains, by use of a reducing agent thereby converting the silver halide to silver and the halide. The unsensitized silver halide grains are dissolved and washed away leaving an original silver particle image of the radiant energy image. For information retrieval using most prior art methods a minimum density above base fog of approximately 0.3 optical density units is required. The method of the present invention may be used for the enhancement of photographic images of even lower optical density.
Various methods for extracting information from underexposed films or radiographs containing a metallic silver particle image are known. One such method involves the deposition of copper on the silver image of a conventionally developed silver halide-emulsion film, as disclosed, for example, in U.S. Pat. No. 3,674,489 issued July 4, 1972. A basic problem with all such methods of intensification is that large amounts of material must be diffused through the gelatin matrix to increase the size of the grains forming the image. Some of this material frequently deposits at places other than the image and spoils the photograph. In addition, if the photographic image includes dense areas, such areas are completely blocked if sufficient material is deposited for image enhancement of the low optical density areas included thereon.
Autoradiographic photographic image enhancement methods also are known wherein the photographic image is made radiactive in an amount related to the the optical density thereof. The radioactive film is placed adjacent a radioactive-sensitive film for exposure thereof to nuclear radiation emitted thereby. Exposure to the radioactive image source continues until the film is properly exposed, after which it is developed. With such nuclear intensification technique, resolution is limited by the resolution of the original negative, the resolution of the autoradiographic film, the evenness of the autoradiographic film contact during exposure, and the range of the radioactive emissions. Since the random direction emissions cannot be conveniently focused, the highest resolution intensification is obtained by contact autoradiography. Additionally, such technique requires specialized equipment and personnel trained in radiochemistry.
Photoluminescence type image enhancement methods also are known as shown, for example, in U.S. Pat. No. 4,299,904 issued Nov. 10, 1981. There, underexposed photographic images are made photoluminescent by photoluminescent material applied to the image. The photoluminescent image then is exposed to photon excitation to excite the same to luminescence. The photon emission then is recorded for production of an intensified image of the underexposed photographic image. Some photoluminescent material is retained by the gelatin of the film and this reduces the image-to-fog fluorescence ratio and can cause serious loss of resolution on thick emulsion radiographs.
An object of this invention is the provision of an improved method and apparatus of photographic image enhancement which avoids many of the above and other shortcomings of prior art image enhancement methods.
An object of this invention is the provision of an improved method and apparatus for reading underexposed photographic films and radiographs which is well adapted for use with silver photographic or radiographic images having such low optical density, or contrast, that they are unreadable by many prior art methods.
An object of this invention is the provision of a method of the above-mentioned type which can be performed such that restoration of the original metallic silver photographic image is possible.
The above and other objects and advantages of this invention are achieved by increasing the reflectivity of the metallic silver image by, for example, converting the image to a silver halide image having substantially the same relative localized optical density as the metallic silver image. The converted image then is placed in a dark-walled cavity which may contain a liquid of suitable refractive index where it is scanned by an electromagnetic energy beam such as a laser beam. Liquid in the cavity, such as water, reduces light reflection from interfaces at the front and back surfaces of the film, and the dark walls of the cavity minimize light reflection therefrom. An electromagnetic energy detector, such as a photomultiplier tube, is positioned in the cavity to receive reflected light from the reflective image. The analog signal output from the detector is converted to digital signal form for storage in memory and subsequent computer processing and display.
The invention will be better understood from the following detailed description considered with the accompanying drawings.